A marine outboard engine generally comprises a bracket assembly that connects the drive unit of the marine outboard engine to the stern of a boat. The drive unit includes an internal combustion engine and a propulsion unit having a propeller. The marine outboard engine is typically designed so that the steering angle and the tilt/trim angles of the drive unit relative to the boat can be adjusted and modified as desired. The bracket assembly typically includes a swivel bracket carrying the drive unit for pivotal movement about a steering axis and a stern bracket supporting the swivel bracket and the drive unit for pivotal movement about a tilt/trim axis extending generally horizontally. The stern bracket is connected to the stern of the boat.
The drive unit's trim angle, and therefore the propulsion unit's drive system, can have a significant effect on a boat's hydrodynamic properties, and hence also have an effect on the engine's performance. When at low speed, the boat sits deep in the water, and is kept afloat by buoyancy. As speed increases, so does the hydrodynamic lift force of the water against the hull. When at a high enough speed, and with a properly trimmed propulsion unit, a boat can get to an “on plane” condition where the boat skims over the surface of the water (i.e. planes) and thereby creates less hydrodynamic drag. Properly trimming the drive unit can allow the boat to cruise at a desired speed at a lower engine speed than if the drive unit is not properly trimmed. Also, properly trimming the drive unit can allow a higher top speed to be reached.
When at cruising speed, trimming in raises the stern and pushes the bow down, while trimming out lowers the stern and raises the bow. When the drive unit is trimmed too far in, the bow ploughs into the water, displacing more water than necessary, thereby increasing drag and slowing the boat. This condition can also create undesirable additional splashing of water by the bow. When the drive unit is trimmed too far out, the stern can be pushed too deep into the water, again increasing drag and slowing the boat. This can also lead to undesirable porpoising where the bow of the watercraft cyclically rises above the water and then dives into the water. Also, should the drive unit be trimmed out to the point where the propeller is no longer submerged in the water, an undesirable condition called “ventilation” occurs that can cause a loss of power and can cause the engine to rapidly increase in speed.
As such, in addition to having to control the steering direction and throttle of an outboard engine, the driver of a boat has to control a trim position of the propulsion unit in order to obtain optimal performance from the outboard engine. Although many boat drivers like to control the trim themselves, other drivers may desire not to have to worry about the trim and focus on steering and throttle.
Therefore there is a need for a method and a system for controlling a trim position of a marine propulsion unit without a direct input of the driver of the boat.